Most of the water on the surface of earth is saline and cannot be used for domestic purposes. In areas where fresh water is scarce there is a need to desalinate salt water. Conventional desalination of water is an energy intensive process. Depletion of fossil fuels, high greenhouse gas emissions and rising costs of energy have necessitated the use of unconventional energy sources. Low grade heat sources, whose temperature is not substantially above 100° C., are one such source. Examples of such low grade heat sources may include hot industrial waste fluids like hot water, steam, exhaust gases, process liquor etc. or renewable energies such as geothermal heat, all of which can be used as heat source or a liquid source to be desalinated.
Desalination can be generally categorized into two types: thermal (distillation) systems like MED (Multi Effect Distillation), MSF (Multi Stage Flashing), HD (Humidification-Dehumidification); and membrane systems like RO (Reverse Osmosis).
Low grade heat sources are particularly suitable for the use in thermal desalination methods. Particularly MED including MED process and Vapor Compression (VC) MED process such as Thermal Vapor Compression (TVC) MED process and MSF systems have been developed to use heat from low grade heat sources. Such systems use a heat exchanger coupled to a low grade heat source to heat saline water which is then fed to either an “effect” of a MED system or a “stage” of an MSF system. In such a system, a substantial amount of heat of the low grade heat source remains unused, as it is not transferred to the saline water. Due to this, energy extraction from the low grade heat source is limited. This further results in a heating medium output that is still hot and the saline water obtained can be used for further desalination.
An improvement over the conventional low grade heat source driven MED system is the so called “boosted MED” system. In boosted MED the unused heat from the low grade heat source is reused to heat saline water, which then introduces the generated vapor into an effect of the MED system. This system incrementally improves the efficiency of extraction of heat from the low grade heat source as well as the yield of fresh water. However, the method fails to extract the maximum possible heat energy from the low grade heat source.
According to the foregoing discussion, there still remains a need to improve the efficiency of thermal desalination processes to make desalination environmentally and financially more viable.